Ion implanters are well known and generally conform to a common design as follows. An ion source produces a mixed beam of ions from a precursor gas or the like. Only ions of a particular species are usually required for implantation in a substrate, for example a particular dopant for implantation in a semiconductor wafer. The required ions are selected from the mixed ion beam using a mass-analysing magnet in association with a mass-resolving slit. Hence, an ion beam containing almost exclusively the required ion species emerges from the mass-resolving slit to be transported to a process chamber where the ion beam is incident on a substrate held in place in the ion beam path by a substrate holder.
Ion beams of different shapes have been used in the past. Ribbon beams are well known and generally have a major axis that is greater in dimension than the substrate to be implanted and a minor axis much smaller than the substrate. Another common type of ion beam is the spot ion beam where the cross-sectional profile of the ion beam is much smaller in all directions than the substrate to be implanted. With either type of ion beam, the ion beam and substrate are moved relative to one another such that the ion beam scans the entire substrate surface with the aim of achieving a uniform ion implant across the whole of the substrate. For a ribbon beam, only one scan across the substrate is required, whereas multiple scans are required for a spot beam. Scanning may be achieved by (a) deflecting an ion beam to scan across a substrate that is held in a fixed position, (b) mechanically moving a substrate whilst keeping an ion beam path fixed or (c) a combination of deflecting an ion beam and moving a substrate.
Our U.S. Pat. No. 6,956,223 describes an ion implanter of the general design described above that uses a spot beam. While some steering of the ion beam is possible, the implanter is operated such that ion beam follows a fixed path during implantation. Instead, a wafer is held in a substrate holder that is moved along two orthogonal directions to cause the ion beam to trace over the wafer following a raster pattern like that illustrated in FIG. 1. First, the wafer is moved continuously in a single direction (the fast-scan direction) to complete a first scan line. The substrate is then stepped down a short distance orthogonally (in the slow-scan direction), and then moved back along the fast-scan direction to form a second scan line across the wafer to overlap with the first scan line. This process is then repeated such that the combination of tracing scan lines punctuated by the stepwise movement results in the whole surface of the wafer seeing the ion beam. The series of scan lines that leads to a complete dosing of the wafer is referred to herein as a “pass”. An implant may comprise multiple passes over the wafer.
Further improvements may be made to improve the uniformity of implants made using such raster scans. For example, multiple passes over the substrate may be made and interlacing may be effected (e.g. make a first pass implanting the first, fifth, ninth, etc. scan lines, then make a second pass implanting the second, sixth, tenth, etc. scan lines, then make a third pass, etc.). Also a problem of angular effects (i.e. off-normal incidence of the ion beam or asymmetries in the ion beam) may be addressed by making multiple passes with rotation of the wafer between passes. For example, in a quad implant four (or a multiple of four) passes are made with a 90° twist of the substrate between each pass. Changing the orientation of the wafer clearly helps alleviate such angular effects. Our U.S. patent application Ser. No. 11/527,594 (U.S. Patent Application Publication No. 2007/0105355) provides more details of such scanning techniques. While such techniques offer excellent uniformity in dosing, the need to perform multiple passes has an associated time overhead that reduces the throughput of the ion implanted.
U.S. Patent Application Publication No. 2001/0032937 describes a very different method of scanning a substrate that does not rely solely on linear movement of the substrate relative to the ion beam. Instead, as illustrated in FIG. 2, a substrate is spun about its central axis with a constant angular velocity, while also being translated across a fixed position, elongate spot ion beam. Movement of the substrate effectively sees the ion beam travel through the centre of the substrate. The ion beam is unusual in that it is nether a conventional spot beam, nor is it a ribbon beam. Rather, it is elongated such that it has a longer major axis that is smaller than the width of the substrate.
As the ion beam first clips the substrate, the rotation of the substrate sees the ion beam implant the periphery of the substrate: as the substrate is translated across the ion beam, the implanted region grows in width and spirals into the centre of the substrate before spiraling out and moving off the periphery of the substrate. However, the linear speed of the edge of the spinning substrate is much faster than the linear speed of the centre of the substrate. To compensate for this effect, the substrate is translated at a variable velocity through the ion beam such that its speed is greatest at the centre of the substrate.
In practice, such a technique is difficult to implement. The control law for the translational velocity is complex and achieving accurate control of this varying velocity is problematic. Worse still, to achieve high uniformity of implant across the substrate requires exceptional uniformity in the ion beam.